Melt butter and add flour to make a paste. Whisk in milk gradually. Add a small amount of milk mixture to beaten yolk and whisk until smooth. Return the yolk/milk mixture to the saucepan and simmer until thickened. Add cheese and season.

Assemble Turkey Hot Brown in an ovenproof dish (individual dishes would be great). Place toasted bread in the bottom of the dish. Add warmed slices of turkey. Pour cheese sauce over the turkey. Add bacon and top with tomato. Broil until cheese is slightly bubbly.